A body of water, especially a freshwater lake, may have a number of problems. First, and perhaps most importantly, may be high acidity caused by acid rain. Unfortunately, treatment of lakes polluted by acid rain has been haphazard. A typical treatment for an acid body of water such as a lake includes "bombing" the water with tons of limestone and then allowing the limestone to settle. Such a treatment creates "hot spots" or warm regions in the lake that may reduce the amount of oxygen in the lake and otherwise adversely affect the ecosystem of the lake.
Second, oil spills have become all too common. Although oil spills caused by ocean tankers running aground may gather headlines, leakage from pipelines or landfills and attendant run off into freshwater lakes or or rivers is perhaps more common.
Third, biological "pollution" may be caused by over enthusiastic attempts to aerate for fishery purposes. A freshwater lake, especially at the end of summer and throughout the fall and winter months, may have an upper oxygen-rich relatively warm layer and a lower oxygen-depleted relatively cool layer. Vigorous aeration of the lower layer is often counterproductive as it may stir nutrients such as nitrogen and phosphorus into the upper layer to produce a conducive environment for algae and other plant growth, which in turn may create plant canopies and block sunlight into the lake.